Modulation of Intersystem Crossing by Chemical Composition and Solvent Effects: Benzophenone, Anthrone and Fluorenone

Combined density functional theory and multireference configuration interaction methods including spin-orbit coupling and spin-vibronic interactions have been used to elucidate the photophysical pathways of benzophenone (BP), anthrone (A) and fluorenone (FL). Our results reveal that the slower S-1((1)n pi*) (sic) T-1((3)pi pi*) passage of FL in the gas phase, as compared to BP and A, originates from the different electronic structures of the T-1((3)pi pi*) states in these compounds and is not related to the planarity of the nuclear arrangement. Temperature effects on the rate constants of intersystem-crossing (ISC), reverse ISC (rISC) and fluorescence have been investigated for FL in solvents of different polarity. Experimentally observed trends for rate constants are well reproduced. A nearly temperature- and solvent-independent, slower down-hill ISC component is attributed to the EI-Sayed-forbidden S-1((1)pi pi*) (sic) T-2((3)n pi*) transition in these environments. The faster EI-Sayed-allowed S-1((1)pi pi*) (sic) T-1((3)pi pi*) ISC requires thermal activation in tetrahydrofuran and acetonitrile solution so that fluorescence may compete against triplet formation. In cyclohexane solution, S-1((1)pi pi*) (sic) T-2((3)n pi*) are nearly degenerate according to our calculations. Hence S-1 (sic) T-2 ISC and S-1 (sic) T-2 rISC have similar rate constants. If T-2 (sic) T-1 internal conversion could be slowed down by deuteration or perfluorination, for example, FL-derivatives might show delayed fluorescence.